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Originally Posted by Selo
What's the next easiest, and hopefully safer way to add horsepower?
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How to get more intoxicating Hp. Man started this quest before Henry built the Model T. Assuming correct air fuel mixture (AFR) and timing, to make more power you need to burn more fuel per unit of time.
An AFR, timing side note. ~14.7:1 AFR is the correct ratio to completely burn all the fuel and have no oxygen left over. Around 12.5:1 makes the most power but leaves un-burned fuel in the exhaust, as it ran out of air. The reason is that the fuel is expanding into a high pressure gas making more pressure even though it is not burned. That high pressure un-burned fuel actually lowers the temp of combustion. WOT at 14.7 AFR will melt engine parts. Here is the important part. Ford engineers target 10:1 AFR at WOT. Why. It saves engines (cooler combustion). They understand they are giving up about 10 Hp by burning too much fuel, but since they cannot control what grade of fuel is put in and how someone drives, it saves them money. Timing is also retarded to assure that the crappiest fuel sold will not go into detonation and destroy the engine. Again they are giving up 10-20 Hp but it saves them from replacing engines. I am assuming from what you said that this is a factory EFI set up. So if you put the highest grade fuel you can get, reliably, into your car and have in tuned on a chassis dyno, a good tunner will be able to get you another 20-30 Hp. All the factory safety factor will be gone, and it is up to you to never dump in a lower grade fuel. If you hear pinging get off the throttle, especially when the engine is hot (running hard).
Personally, my butt cannot feel much less than 25 Hp change. I would save the tunning until after I slipped a cam in or added better heads or both.
Back to burning more fuel per unit of time. Since the AFR has to be correct, you have to pump more air through the engine to burn more fuel. Hence the old adage, there is no replacement for displacement. The more cid the more air you pump. There is an alternative to displacement, Forced Induction. Turbo charger, centrifugal blower, lobed blower, screw compressor, these will all push more air into the engine. If you bring the intake manifold up to 14.7 psig you will have double atmospheric pressure (at sea level). You can then burn twice the fuel and make twice the power, minus whatever power was consumed to power the air pump used.
High boost levels has many issues that has to be done right or the engine is easily destroyed. Lot's of heat is added to the air that if not delt with, will auto ignite the fuel before the plug fires. Ford has mastered this with the turbo charged ecoboost engines. Today the standard tiny 4 cylinder engine in the Mustang makes more Hp than the GT with the old 5.0 or 4.6 modular.
There are many forced induction options that will bolt on to your 5.0, but fitting them into a Cobra can be a challenge. A mild 5-8 psi boost would get you well above the factory 225 Hp. North of 300 fairly easy. Getting to 400 hp without changing internal engine components is not very realistic. Well it would run for a while and then you could replace what is left of the engine.
Someone else mentioned stroking the engine to 347 cid (which includes 0.030" over bore). You cannot go wrong there. No replacement for displacement. Good heads make the engine. Factory heads are a huge bottle neck in the power making department.
Understand that your engine likely has 19 Lb/hr injectors. To burn more fuel you will need bigger injectors somewhere north of 300 Hp. Not too much north of that you will need bigger fuel rails and a bigger fuel pump. Your fuel lines may be too small too.
Also understand that if you stroke the engine and put good heads on it, the existing intake manifold and throttle body will be too small. If you go forced induction they can stay.
The biggest point I can make is that if you start adding a piece at a time to make a little more power here and there, start with an end goal in mind. The best heads for a 302 is not the best for a 347. Someone may recommend a great head for your 302 but if you are going to stroke it to 347 better to buy those heads, unless you can toss $2000 to a set of heads you are only going to run for a while and the toss another $2000 again later.
Food for thought anyway.